1. Field of the invention
The present invention relates to a vehicle alternator to be mounted on vehicles such as a passenger car and a truck.
2. Description of the Related Art
It is widely known to generate a higher harmonic magnetic field in a space between a stator and a rotor in a vehicle alternator by the reaction of an armature. Further, it is also known to generate a small local-variation of magnetic filed, called “a slot ripple”, in a rotary angle. Such a generation of the slot ripple is based on the variation of a magnetic field resistance between opposed areas faced to each other between the stator and the rotor. Such an alternating magnetic field generated by the variation does not have a bad influence on the stator made of laminated plates, but has an adverse effect of eddy current loss and a hysteresis loss to magnet poles of the rotor made of an iron ingot.
Recent trend on a vehicle technology has strong demand of reducing the fuel consumption of a vehicle, and for this reason, there are necessities of enhancing the performance of the vehicle alternator and of reducing the eddy current loss in the vehicle alternator. In order to meet such strong demands and necessities, conventional technologies, for example, Japanese patent laid open publications JP H3-139149 and JP H5-83909 have disclosed a vehicle alternator having uneven portions formed on surfaces of magnet poles of a rotor in a vehicle alternator. The formation of those uneven portions can prevent the generation of an eddy current.
Although those conventional techniques is capable of eliminating the eddy current loss caused by eddy current flowing through the magnet poles of the rotor in the vehicle alternator, the ratio of the area of the uneven portions to the surface area of the magnet poles becomes larger because those uneven portions are formed on the entire surfaces of the magnet poles of the rotor. If the interval between the surface of the magnet pole of the rotor and the surface of the stator is increased by the formation of the uneven portions, an average length between them is increased and this condition reduces the magnitude of magnetic flux and also reduces the output of the vehicle alternator.